In general we are currently investigating water and electrolyte movement in the small intestine and the colon in vivo in both healthy controls and patients with chronic diarrhea. To do this we use a steady state intestinal perfusion technique which employs the triple lumen tube and nonabsorbable volume markers in perfused test solutions. Specifically, in normal subjects we are examining the normal transport process of water and electrolyte movement in the colon, examining the mechanism of the VIP effect on water and electrolyte movement in the jejunum, and examining the effect of codeine on normal intestinal absorption and VIP-induced secretion. In chronic renal failure patients we are evaluating the effect of 1,25-Dihydroxyvitamin D on phosphate absorption.